My plattys were ok at 28-30 degrees when I had to treat tank for whitespot. It stayed at this temp for 10-14 days without problems.
I used Interpets whitespot meds as instructed & it cleared problem.
Don't know how the Harlequin rasboras would cope with this temp but dwarf gurami, corys, neon & CAE wre all ok so as long as the increase/decrease isn't too rapid it should be ok for them.
Good luck.
PS use an air pump & stone as temp & meds will decrease oxygen in the water.